There are three major phases in paper manufacture where starch is used as an ingredient. The first is the "wet end" where cellulose fibers are mixed with starch in a slurry, and the slurry is forced through a narrow opening onto a wire belt. Water is rapidly removed as the forming sheet travels the length of the belt. After a distance of typically five to fifteen meters on the belt, the sheet has had enough water removed from it so that it can support its own weight. The sheet travels through a number of foils and rolls wherein more water is removed. It is dried to about 11% moisture.
The second phase in paper manufacturing involving starch is the "sizing step". Here, the paper goes through a sizing press where a slurry including starch is applied to the sheet. The sheet again goes through a series of foils and rolls. It is dried on rollers and can be taken off the press as a finished product.
The third step involves coating the paper with a mixture of starch and a thermoplastic molecule. On certain lines, this occurs after the sizing step. The nascent roll can also be removed and reinstalled onto a different press for coating. A typical coating device has two blades that run the width of the paper. The blades apply the coating material onto two rolling drums. The paper passes between the drums and the coating material, comprising starch and the thermoplastic moiety, comes off the drums onto the paper. After the paper leaves the drums, it goes through a number of dryers. When the paper is dry, it goes onto a "soft calendar" comprising two drums, one made of a hard density fabric and the other a heated steel drum. The paper passes between the two drums and the heated steel drum is sufficiently hot to melt thermoplastic components of the coating mix providing a hard gloss finish on the paper.
The cellulosic wood pulp fibers, typically used in the above process, are anionic in nature. The addition of a cationic starch to the "wet end" slurry acts as an adhesive by cross linking the pulp fibers through salt linkages. Thus a cross linked polymeric network is made, comprising the starch and cellulose fibers. Typically, the cationic starches used in the "wet end" contain tertiary or quaternary amines. These amino groups are added to the starch following the wet milling process.
Surface sizing starches are used to impart both strength and smooth finish to the sheet after it leaves the "wet end". Such starches also prepare the sheet to receive the various coatings. In cheaper grades of paper and in fiberboard manufacture, sizing starches are used simply as unmodified corn starch. For high grades of paper, chemically-modified starches are used. This is important for the application of a smooth, uniform high quality surface to the paper.
There is a tendency for starches to retrograde i.e. re-form high ordered structures (both helices and crystallites) in an otherwise gelatinous starch slurry. Deposition of retrograded starch onto high quality paper causes regional inconsistencies on the paper and is unacceptable. Furthermore, retrograded starch in the sizing press may necessitate shutting the line down to clear the apparatus.
The starch most often used for sizing applications is a starch having a covalently attached neutral adduct, for instance hydroxyethyl starch. This is prepared by the reaction of ethylene oxide with starch after it is isolated at the wet milling plant. The function of the hydroxyethyl (or similar) adduct is independent of its chemical nature; rather, it serves to provide steric hindrance, inhibiting the formation of high ordered structures. This steric hindrance is critical to decrease retrogradation. The periodic protuberance afforded by the adduct disrupts the formation of higher ordered structures that leads to retrogradation.
Speed is of paramount importance in paper manufacturing. What limits press speed is the requirement to remove water. With a higher concentration of starch, there would be less water to remove, and the press could run at higher speed. However, higher concentrations of starch accelerate retrogradation and retrograded starch deposition onto the sheet and as noted above is unacceptable.
Hydroxethylated starch also forms higher ordered structures as the temperature decreases or the concentration increases. The formation of the higher ordered structures on the surface of the paper is required. After application to the sheet the starch reforms some of these higher ordered structures and creates a uniform surface that imparts structural strength and facilitates the acceptance of inks and dyes. However, the higher ordered structures should not form in the slurry nor on the application device because this necessitates shutting down the production line to clear off retrograded starch.
The function of the hydroxyethyl group is to lower the temperature and/or raise the concentration of starch at which retrogradation occurs. As the processing lines have already been optimized for a particular temperature of the starch slurry, a decrease in the tendency to retrograde would allow for a higher carbohydrate content in the slurry.
The mixture applied to the paper sheet in the coating process contains hydroxethylated starch and thermoplastic molecules. The most prevalent thermoplastic molecules used are latexes, such as styrene butadiene. The function of the hydroxethyl starch is as indicated above. The function of the thermoplastic molecule is to form a high gloss finish on the paper. This causes an increased ability to take inks and dyes and improves the resolution, in general, on the printed sheet.
Based on the foregoing, there exists a need, in paper manufacturing, for modified starch substitutes which are functionally similar to modified starch. There is a further need to provide substitutes for modified starch which are less prone to retrogradation. There is a further need to provide methods of manufacturing paper which are faster than current methods and allow presses to run closer to their full capacity speed. There is a further need to provide methods of manufacturing paper that are environmentally-friendly and do not involve input materials that require chemical processing. Meeting these needs would advance the state of science and industry in this area.